[Mesa-dev] [PATCH v2] i915: fixing driver crashes if too few vertices are submitted

[Ian Romanick]

On 09/09/2015 11:53 AM, Ian Romanick wrote:
> On 09/09/2015 11:16 AM, Marius Predut wrote:
>> Comparison with a signed expression and unsigned value
>> is converted to unsigned value, reason for minus value is interpreted
>> as a big unsigned value. For this case the "for" loop
>> is going into unexpected behavior.
>>
>> v1:Brian Paul: code style fix.
> 
> I don't think you really did...
> 
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38109
>> Signed-off-by: Marius Predut <marius.predut at intel.com>
>> ---
>>  src/mesa/tnl_dd/t_dd_dmatmp.h | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/mesa/tnl_dd/t_dd_dmatmp.h b/src/mesa/tnl_dd/t_dd_dmatmp.h
>> index 7be3954..79de224 100644
>> --- a/src/mesa/tnl_dd/t_dd_dmatmp.h
>> +++ b/src/mesa/tnl_dd/t_dd_dmatmp.h
>> @@ -627,6 +627,9 @@ static void TAG(render_quads_verts)( struct gl_context *ctx,
>>        LOCAL_VARS;
>>        GLuint j;
>>  
>> +      if(count % 4 != 0)
>            ^
> ...because I'm quite sure Brian's code had a space here, per Mesa's
> coding standards.
> 
> Also, this change is incorrect.  If an application does
> glDrawArrays(GL_QUADS, 0, (n * 4) + 3), this change will cause zero
> quads to be drawn when n quads should be drawn.
> 
> Page 18 (page 32 of the PDF) of the OpenGL 2.1 spec says:
> 
>     "The total number of vertices between Begin and End is 4n + k,
>     where 0 ≤ k ≤ 3; if k is not zero, the final k vertices are
>     ignored."
> 
> We probably don't have a piglit test for this scenario, so one should be
> added.  You can CC me on that patch. :)
> 
> I think the correct change is to trim count such that (count % 4) == 0.

Also... wherever you add this trim code, you should include the spec
quotation that I mentioned above.

>  If the modified value of count is zero, bail out.  With that change,
> the other hunk (below) is unnecessary.
> 
>> +          return;
>> +
>>        INIT(GL_TRIANGLES);
>>  
>>        for (j = start; j < count-3; j += 4) {
>> @@ -1248,7 +1251,7 @@ static GLboolean TAG(validate_render)( struct gl_context *ctx,
>>  	    ok = (GLint) count < GET_SUBSEQUENT_VB_MAX_ELTS();
>>  	 }
>>  	 else {
>> -	    ok = HAVE_TRIANGLES; /* flatshading is ok. */
>> +	    ok = HAVE_TRIANGLES && (count % 4 == 0); /* flatshading is ok. */
>>  	 }
>>  	 break;
>>        default:
> 
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